Paving stones of the type to which the present invention relates are manufactured slab elements usually molded of ceramic material, most commonly concrete, into predetermined shapes which, when arranged in a pattern, form a covering for the ground or other surface area which is generally intended to bear pedestrian or vehicular traffic.
Bricks, cut stones and slab elements of various types have been used in the past to cover roads and walkways to form a pavement or ground cover arrangement. In forming the ground cover pattern, the elements are often laid adjacent each other in an array to cover the area being paved. The most common shape of element used historically is the rectangular brick like shape which can easily be arranged to cover the ground without resort to combinations of stones of different sizes or shapes to do so. Such elements are laid with or without grout or mortar joints which rigidly join one element with another.
A type of ground cover finding increasing use is that formed of the paving stones laid without mortar or grout, usually with joints filled with particulate material such as sand. The advantages which such ground covers present are an ability to tolerate movement and deformation without exhibiting the cracking and breaking which may result with ground covers in which rigid grout or mortar joints are employed.
An example of a disadvantage found with some paving stones of the prior art, as for example the simple rectangular elements such as bricks and rectangular stones, is that, when used with sand or other loose fill joint material, surface water flowing on the pavement area formed of such a ground cover has a tendency to erode or wash the joint material from between the elements. A further example of a disadvantage of many such elements is that they have a tendency to tilt or yield under locally heavy loads.
One solution to both the problem of the washing of joint material from between the elements and to the problem of movement under load has been the introduction of mortarless or groutless paving stones of the interlocking type. Such interlocking paving stones are for example those disclosed in the Hair U.S. Pat. Nos. 4,544,305 and 4,973,192 and of Barth et al. Nos. 4,128,357.
An objective in the design of interlocking paving stones, as seen in the Hair and Barth patents, is the creation of shapes which will interlock in such a way as to cover the area being paved with a minimum of different stone shapes. It is highly desirable that stones of a single size and shape be capable of forming an interlocking pattern which covers the ground without the need for filler stones of different shapes. Such a characteristic reduces the number of costly molds and the need for distributors and installers to maintain inventories of different stones.
One disadvantage of most of the paving stones of the prior art, when laid without mortar or grout filling the joints, is that the drainage area between adjacent stones is narrow, typically 3/8 or less, the width of a typical mortar joint. This closeness is necessary to that the adjacent faces of adjacent stones provide structural support to each other, to hold the stones in place and level. However, for certain applications, such spacing is inadequate to provide the necessary drainage that the site requires. While it is possible to lay many stones of the prior art with open spaces present in the patterns to allow additional drainage, many of such stones do not at the same time interlock, and thus form a ground cover that is less effective in resisting distortion and breakage with heavy loads.
Other stones of the prior art are provided with shapes to facilitate drainage. An example is the stone described in the Welling U.S. Pat. No. 4,997,308. Such a stone does not, however, have interlocking capabilities.
A desirable feature of paving stones is an ability to be laid in a herringbone pattern. The herringbone pattern, with its crossing stone orientations, provides a stronger ground cover, more effective interlock between stones, and better resists erosion of the loose material from between the stones. Many stones of the prior art, such as the stone of Welling, discussed above, do not lend themselves to arrangement in a herringbone pattern.
It has also been an objective, difficult in many cases to achieve, to shape the stones in a way that they will not only satisfactorily interlock and form a pattern which covers the area being paved, but which will do so with shapes which present boundaries which contribute to a particular aesthetic pattern. By the very nature of the stones, the boundaries that define their shapes make the primary contribution to the overall appearance of the patterns. Unfortunately, not all aesthetically desirable shapes are easily made to interlock effectively. The desire to provide certain shapes in paving stones makes it difficult to design stones which interlock effectively. Thus, the desire to form patterns which yield certain aesthetic effects imposes a constraint on the stone characteristics which preclude the utilitarian properties for which the interlocking stones are desired.
In addition, many paving stones of the prior art have, when attempting to achieve the aesthetic and interlocking pattern forming objectives, failed to produce a stone that is capable of bearing heavy loads and resisting breakage.
Accordingly, there has existed a need for an interlocking paving stone with sides angled and shaped to cover the ground with stones of a single size and shape, which are sufficiently strong to gear heavy loads, which can be laid to provide adequate drainage for the requirements of the site, and which can be laid in open patterns, and particularly open patterns including a herring bone pattern, to provide drainage area, and which can do so with a stone of a single shape.